This invention relates to welding and more especially to welding of metal usually but not invariably ferrous metal in the form of plate or structural members.
A common welding requirement, with which the present invention is particularly concerned is the welding of one plate to another, at, or almost at, a right-angle configuration, i.e. a "T" weld.
Typically, this is effected by creating a weld "fillet" along each internal corner, which necessitates two runs. The need to angle or bevel the attached plate corners to improve weld penetration also arises if heavy gauge plate is used. Similar techniques are used when a rolled or fabricated section is to be attached by welding to a face of a plate.
The deep penetration characteristics of high-powered laser beams have led them to be considered as a source of energy in welding processes. Our copending Application based on British applications Nos. 83 10630, 83 10631 and 83 10632 (now U.S. Pat. No. 4,634,832, issued Jan. 6, 1987); entitled "Laser-Bremwelding" describes such a process particularly adapted to use in workshops or site conditions. The process involves the use of an impingement member such as a wire so that any gaps in the fit-up between the plates to be welded do not lead to a loss of energy due to escape of the focussed laser beam through the gap, i.e. without energy transfer and plasma formation. The method of the prior invention can be applied both to butt welding and to T-welding.
The present invention sets out to provide a novel technique of welding utilising the deep penetration of a finely focussed laser beam to achieve melting and welding of a T-section, or like, weld under actual workshop or site conditions, in which the members to be welded are not necessarily so accurately prepared as to provide a tight-welding fit, nor so thin grade material that they can be forcibly damaged into tight abutment, but from time to time may exhibit gaps along the weld line to be produced.
In one aspect the invention consists in a method for welding a metal plate to an underlying attachment member, which in different parts of a mutual attachment region can be contacting or in close but non-contacting proximity to the plate: contacting or in close proximity to the plate: in which a high energy laser beam is focussed upon the plate and is moved along relative thereto at the other side of the plate from the said mutual attachment region contact area, characterised in that a supply of gas is directed to at least partially confine the plasma formed, whereby the focussed beam melts the metal both of the plate and of the underlying member so that upon solidification a weld line is formed, and in that a filler wire is provided for feed into the focussed beam at a rate required to fill any gaps between the plate and the member.
Usually, both the plate and the underlying member are ferrous. By "plate" is generally understood material at least 3 mm thick; however, thicknesses to as thin as 1 mm may also be used. We have found that from 3 to 25 mm is preferred and that a laser of 5 to 25 kw intensity can be utilised.
The underlying member is expected to be spaced from the plate in some places and not others. The filler material is valuable since when molten it will flow to locally fill any such space to improve weld characteristics, especially by filling any gaps open to the outside, where corrosion can start if necesssary by making a number of through welds.
The relationship between the plate and the underlying attachment member can be that of a "T" weld, i.e. a weld where the underlying plate is placed at, or nearly at, right-angles to the upper plate. The weld then passes into the end face of the underlying plate, in a single deep penetration pass, giving a "through-welded" joint of unique form.
An alternative relationship is that where the underlying attachment member has been formed to contact as a lip or margin parallel to the underside of the upper plate. The width of the formed lip may permit two or more parallel "through welds" according to the invention to be formed.
Another alternative relationship is of course that of two overlapping but generally parallel plates. Other relationships can also be envisaged, as shown in more detail below.
The upper plate utilized for a T-weld is more preferably from 2-15 mm in thickness for laser power up to 20 kw; the underlying plate may be of any thickness.
We are aware that GB Pat. No. 1 268 044 describes a laser welding process which pierces through an upper sheet of material, usually as spot welding. The process therein described utilises the technology then available, i.e. lasers of lower power, and carries out the process on thin sheet. It recognises an inherent limitation in the process, namely that the power density of the radiation must be kept below a ceiling value to avoid vaporisation to such an extent that there is insufficient material left to form a weld. Moreover, it is concerned with thin material, 0.38 mm sheet being given by way of example, in which thermal conduction problems, and weld characteristics, are different from those obtainable with thicker material. Thus, application of the process of this prior Patent to thicker grades of material is contraindicated.
Thicker grades of material also present the problem that when superposed they can exhibit gaps due to inevitable minor imperfections in cutting or shaping, and that it is unduly laborious and expensive except under laboratory conditions to remove these gaps, e.g. by machining or clamping of such heavy grades of metal into perfect face-to-face contact. In particular, a continuous weld (as compared to spot welds as described in the G.B. Pat. No. 2 268 044) is required to traverse and join some portions of the mutual metal-to-metal contact areas which are in good close contact and others which are spaced apart to a variable extent. The present invention achieves this by feed of a continous length of material such as wire, provision being made for varying the effective speed of material feed as needed to provide molten material to locally fill the gaps as, if, and when encountered.
Feed of such lengths of material e.g. wire also provides a further advantageous feature of the invention. The laser beam is focussed on the support plate and melts a hollow conical shape with molten walls down through this plate into the underlying structure. As the beam is traversed this shape, called a "keyhole", moves along. If the plate and underlying structure are in close contact the "keyhole" stays the same shape as it moves, leaving in its wake a solified weld structure. If a gap arises between the plate and the underlying structures, some of the molten metal will run from the keyhole walls into the this gap. This changes the keyhole internal shape, and thus the location of vapour or plasma zones in and above the keyhole. Consequently, impingement of the beam is altered and heat transfer to the keyhole is changed. However, feed of the filler material to a position at or near the beam focus means not only that additional molten material is provided, and can flow to fill the space but also that the beam impingement on the wire remains relatively constant, with improved uniformity in operation.
It will be seen therefore that, as in the invention of our copending application Ser. No. 601424 referred to above, gas supply to the weld zone, which is of crucial importance, will act upon a more uniform generation of vapour and/or plasma to achieve its controlling function.
When its form of maximum penetration is needed, according to the present invention, high-powered lasers are being used to the limits of their performance. A balance of welding speed and control of plasma escape is essential, on a considerably larger scale than hitherto, and this is provided by gas supply by means of which a trade-off between speed and depth can be achieved.